This invention relates to inhibitable trigger circuits and particularly to oscilloscope trigger circuits inhibited during a holdoff state.
Devices making use of a trigger signal to initiate a function or activity typically inhibit generation of the trigger signal during the function or activity and for a time afterward, thereby allowing a recovery period between each triggered function or activity and receipt of the next trigger signal.
In a device such as an oscilloscope or digitizer, a display sweep is initiated by a trigger signal presented to a sweep generator. Following the display sweep, a recovery period, known as the holdoff period, is required to allow time for the electron beam to return to the left side of the display screen, i.e., to retrace, and to allow time for the sweep generator to stabilize and prepare for the next display sweep. Following the holdoff period, the oscilloscope is ready for the next display sweep.
The trigger signal is typically generated using an input signal from a predetermined trigger source. The trigger source signal may be internal or external to the oscilloscope; it is typically periodic and often bears some synchronous relation to the signal to be displayed. The trigger source signal is, in any case, a stream of potential trigger points, each a proper candidate for initiating a trigger signal.
The trigger circuit has two fundamental inputs, a trigger source signal and a holdoff signal, and one fundamental output, the trigger signal. The trigger circuit must make a decision to either accept a trigger source signal event or reject the same and wait for the next trigger source signal event. During the display sweep and subsequent holdoff period all trigger source signal events are ignored by the trigger circuit and no trigger signal is generated; after the holdoff period, the next trigger source event presented to the trigger circuit is accepted and the trigger circuit generates a trigger signal.
It is desirable to generate trigger signals bearing a consistent relationship to the selected trigger source event. A consistent relationship is maintained when a trigger source event is presented to a trigger gate at a time when the enabling signal is clearly in the enabling state. This situation is called "early holdoff" because the enabling signal precedes the trigger source event. The resulting trigger signal is a direct result of the presentation of the trigger source event to the trigger gate and, accordingly, bears a consistent relationship to the selected trigger source event.
Interference between the enabling signal and a trigger source event may produce a trigger signal departing from the aforementioned consistent relationship. Presentation of an enabling signal to the trigger gate during presentation of a trigger source event, termed "late holdoff", may corrupt the trigger-source/trigger-signal relationship. It may be said that, in the case of late holdoff, the resulting trigger signal is caused more by the advent of the enabling signal than by the occurrence of the trigger source signal event; such a trigger signal does not bear a consistent relationship to the trigger source signal.
The term "jitter" refers to the horizontal displacement of the waveform portrayed on an oscilloscope display caused by the inaccurate or inconsistent timing of the trigger signal in relation to the selected trigger source signal event. Existing trigger circuits suffer from excess jitter caused by the interaction of the trigger source signal event and the inhibition, or holdoff, signal. The present invention serves to greatly reduce jitter caused by interaction between the trigger source signal and the inhibition signal, without limiting the trigger bandwidth.